Curable resins, such as, epoxies, acrylates or methacrylates, maleimides, vinyl ethers, and bismaleimides, are studied for use as die attach adhesives in semiconductor packaging applications (adhesives used to attach a semiconductor die to a substrate) because of their low moisture absorption, good curability, and long pot life. Two other desirable properties for die attach adhesives are (i) flexibility at room temperature and (ii) adequate cohesive strength at elevated temperature.
Flexibility at room temperature is desirable to absorb the stress in the die and substrate created by their differing expansion and contraction rates in response to temperature during fabrication processes. Flexibility is measured as modulus, the higher the modulus, the less flexibility. High modulus at room temperature translates into increased stress in a semiconductor package and can lead to delamination and die cracking. Therefore, the lower the modulus at room temperature, the better the flexibility and material performance.
High temperatures can degrade adhesives without adequate cohesive strength. If die attach adhesives do not have sufficient cohesive strength to withstand the elevated temperatures encountered during processing and use, the result can be delamination of the die from the substrate and ultimate failure of the semiconductor device in which the die is used.
One method for obtaining flexibility in die attach adhesives at room temperature consists in using components that give low levels of crosslinking. In counterpoint, a method for improving cohesive strength at elevated temperatures consists in using liquid components that give high levels of crosslinking; but this approach is even not always effective because although the crosslink density is increased, cohesive strength at elevated temperatures can remain unaffected.
This invention comprises a solution to the above problem by providing a composition that exhibits improved cohesive strength at elevated temperatures with minimal decrease in room temperature flexibility.